Showers Pass makes waterproof socks. I've used them for snowshoeing as well as trail races that had numerous water crossings. I haven't had a chance to try them out on a bike ride yet, but I bet if can make sure that I've covered the tops of the socks with my water resistant pants that my feet would stay dry.

100% agree about fenders! Just acquired a new bike suitable for brevets that has full length fenders. Kept me dry on a 100K ride last month that had slushy road conditions for the first hour or so. On that ride, I had on my winter cycling boots (Lake 303) which kept my feet warm and dry even though temps never got above 33 (started at 21 degrees).